Three Year Evaluation of High Populations of Reniform NematodeOn Yield and Quality of Sweet Potatoes in the Mississippi Delta

Larry Adams and Craig Abel IIntroductionThe reniform nematode, Rotylenchulus reniformis,can cause significant losses in sweet potato production through reduced yield and cracking damage. This has been documented across the historical sweet potato producing areas in the United States.Recently, Mississippi delta growers turned to alternative crops to diversify their operations. Sweet potato was a crop that delta producers felt would be profitable on their farms. In 2003, USDA, ARS, SIMRU at Stoneville, Mississippi, started a program to study pest populations and damage in sweet potatoes grown in the Mississippi delta. Traditionally, high populations of nematodes are encountered in the sandy/silt loam delta soils that are suitable for sweet potato production.In 2003, SIMRU began a two year study of the effects of reniform nematodes on sweet potato quality and yield and completed a nematicide study during the 2005 growing season.

ResultsResults of USDA, ARS, SIMRU three year study showed substantial increase in yield from soil sampling and control of the reniform nematode.In 2003, an early spring population of 8000 reniform nematodes per pint of soil on a section of our research farm offered the Southern Insect Management Research Unit at Stoneville, MS an opportunity to study the effects of the reniform species, Rotylenchulus reniformis, on sweet potatoes in the Mississippi delta.The test illustrated that, although no apparent nematode damage was recorded to foliage growth or the harvested potatoes, there was a growth stunting effect of the harvested potatoes in non-treated plots. Less root mass and a small amount of gall formation was observed on the fibrous roots. No cracking was observed in the harvested potatoes and no nematode damage was evident in sliced samples. Damage from other sweet potato insects was less than a rating of 1 from our 2003-2005 damage scoring scale (Figure 1). The 2003 plots were harvested early, 90 days after transplanting, which resulted in a low number of US#1 potatoes compared to the 2004/2005 yields, harvested at ~100 days, but all three years showed an increase in yield when reniform nematodes were controlled (Table 1).Initial nematodes numbers were much higher in 2003 compared to the 2004 and 2005 samples (Figure 2). No tillage treatments, other than a Lorsban 4E spray and incorporation by re-hipping in April, were applied in either year to prevent movement of nematodes from non-treated to treated plots. Time between pre-harvest and post-harvest samples in 2004/2005 was approximately 2 1/2 months. The post-harvest sample numbers varied, most likely, due to the disturbance of the soil during harvest. Early harvest of treated plots in 2003 contributed to the low yield numbers but all three growing seasons demonstrated losses due to the presence of a high population of the reniform nematode (Figure 3 and Figure 4).

SummaryDuring these three growing seasons, a significant yield response resulted from controlling threshold levels of the reniform nematode.In the Mississippi delta, pre-harvest or post-harvest soil samples should be pulled from undisturbed rows in sandy/silt loam fields that were traditionally planted in cotton to detect reniform species presence and population.Along with nematicides, a one to two year rotation to resistant crops such as corn, grain sorghum or some varieties of soybeans is another effective tool in keeping populations of reniform nematodes under damaging levels.

AcknowledgementWe thank Chris Johnson and Owen Houston, USDA, ARS, SIMRU, Salliana Stetina, USDA, ARS, CGPRU, Pathologist and Debbie Boykin, USDA, ARS, Statistician, for their assistance with this study.

2003-2005 Sweet Potato Damage Scoring ScaleSample = 30’ of Row Harvested Rating Scale is 0-3: 0 - No damage seen in the sample 1 - 5-10% of the potatoes in the sample are damaged 2 - 10-25% of the potatoes in the sample are damaged 3 - > 25% of the potatoes in the sample are damagedNote: Damage rated in the 2003-2005 samples was from wireworm, armyworm and grub feeding.

Figure 1

Two Year Reniform Nematode Study inMississippi Delta Sweet Potatoes – 2003/2004

USDA, ARS, SIMRU

L. C. Adams USDA, ARS, SIMRUStoneville, MS

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2003-2005 Population Counts of Reniform Nematodes in Mississippi Delta Sweet Potato Research Plots

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L. C. AdamsUSDA, ARS, SIMRUStoneville, MS

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Table 1

Nematode damage to fibrous roots, harvested and stored sweet potatoes

Materials and MethodsThe test plots were transplanted in silt loam soil following several years of cotton with a high infestations of the reniform species, Rotylenchulus reniformis.In both studies the Beauregard variety was transplanted using 4 row 30’ plots during the middle of May. In 2003 soil samples were taken in early spring and post harvest. During 2004/2005 we pulled samples in early spring, mid-season, pre-harvest and post harvest. All samples were pulled from the middle of the row using a standard soil sampler that was cleaned between plots. The 2003 plots were harvested at 90 days and the 2004/2005 plots were harvested at ~100 days. In the two year study Telone II was delivered into the treated plot rows at 6 GPA in early April followed by an application of Lorsban 4E @ 4 pts./A in both the treated and control plots. In 2005 the nematicides Temik 15G @ 10#/A plus Lorsban 4E @ 4 pts./A and K-Pam HL @ 8 gal./A plus Lorsban 4E @ 4 pts./A were used along with a treatment of Lorsban 4E @ 4 pts./A alone and an untreated control. Fertilization, herbicides and insecticides were applied as needed to all plots during the growing seasons. Insect populations were monitored weekly using a prototype hand vac. Yield, insect damage rating and nematode damage was taken and recorded at harvest.

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2005 Nematicide Trial in Mississippi Delta Sweet Potatoes USDA, ARS, SIMRU

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Sample sweet potatoes harvested from 2005 nematicide study plots

Figure 4

Three Year Reniform Nematode Study in Mississippi Delta Sweet Potatoes (2003-2005)

USDA, ARS, SIMRU Mean lbs. per Sample** 50# 50# 50# 2003* bu/A 2004* bu/A 2005* bu/A US1 - TeloneII 11.1 97 27.6 240 Temik15G 21.5 187 K-PAM HL 16.8 146 Lorsban 8.7 76 16.6 145 12.3 107 UnTrt 7.0 61 Canner - TeloneII 16.5 15.6 Temik15G 14.5 K-PAM HL 18.5 Lorsban 13.0 15.8 12.8 UnTrt 12.3 Jumbo - TeloneII 4.8 2.3 Temik15G 3.3 K-PAM HL 1.3 Lorsban 3.3 .5 0 UnTrt 0 MKTotal - TeloneII 32.3 46.0 Temik15G 39.3 K-PAM HL 36.5 Lorsban 25.0 32.9 25.0 UnTrt 19.3

*2003 plots harvested at 90 days and 2004/2005 plots harvested at ~100 days. **Sample size equal 30 feet of row.